I've actually always found C.S. Lewis to be one of the single most fascinating and compelling Christian writers. Obviously I think he makes some very fundamental mistakes, but his approach to Christianity is about as rationalist as you can get. He really emphasizes that if you're going to believe in something, it better really be true not just "worth believing in" or "virtuous" or "helpful" -- he himself could have written Belief in Belief. Furthermore, he seems committed to a conception of "faith" that doesn't involve any conflict with rationality -- he thinks that the logical arguments for the existence of God do a lot of work, and he's fairly sophisticated scientifically (seems reasonably knowledgeable about evolution, quantum mechanics, etc.). I would actually highly recommend The Screwtape Letters to any rationalists who find religious arguments interesting (if not compelling).